A light-emitting device is a so-called organic electroluminescence device in which an organic compound placed between a cathode and an anode emits light owing to an electric current flowing between both the electrodes.
FIG. 1 shows a general sectional structure of a light-emitting device. In the figure, reference numeral 1 denotes a transparent substrate; 2, a transparent electrode; 3, a hole transporting layer; 4, a light-emitting layer; 5, an electron transporting layer; 6, an electron injection layer; and 7, a cathode.
In the light-emitting device, recombination between an electron, which is injected from the cathode 7 into the light-emitting layer 4 through the electron transporting layer 5 and the electron injection layer 6, and a hole, which is injected from the transparent electrode 2 into the light-emitting layer 4 through the hole transporting layer 3, generates an exciton. The light-emitting device utilizes light emitted when the exciton reverts to its ground state.
Used for the cathode 7 of the light-emitting device is a material with a relatively small work function and a good electron injection characteristic, which includes an element metal such as magnesium (Mg) or a metal alloy such as Ag—Mg or Al—Li.
In addition, U.S. Pat. No. 6,013,384 discloses a structure in which an organic layer containing a metal that functions as a donor (electron donative) dopant is placed to be adjacent to a cathode. U.S. Pat. No. 6,013,384 also discloses metals each of which is used as the donor (electron donative) dopant such as alkali metals, alkali earth metals, and transition metals including rare earth.
U.S. Pat. No. 6,013,384 also discloses a structure in which an organic layer containing as a dopant a metal oxide or a metal salt is placed to be adjacent to a cathode.
An organic compound used for each of those organic layers may be, for example, bathophenanthroline, which has a good electron transporting characteristic.
The largest problem in such a light-emitting device is a service life when the device is continuously driven. In particular, the above bathophenanthroline has a glass transition temperature (Tg) as low as about 75° C. Therefore, in a light-emitting device using a material such as bathophenanthroline for its organic layer, the shape of a thin film constituting the organic layer changes owing to heat generation upon the drive of the device. The change in the shape of the thin film is a factor causing inhibition on carrier injection from an electrode, an increase in driving voltage, and generation of a non-light-emitting portion called a dark spot. For such a reason, a light-emitting device using bathophenanthroline exhibited good initial characteristics in terms of emission luminance, emission efficiency, and the like, but had a short service life upon continuous drive.